Has warming spread Lyme?

Sunday, August 14, 2011from Poughkeepsie Journal

The Lower Hudson Valley has long been considered the epicenter for Lyme disease in New York state. As a result, most Dutchess County residents are well acquainted with the disease.

They know it's spread by a tick bite, they perform tick checks after spending time in the forested and brushy habitats ticks prefer, and they consult with a physician if they develop a bull's-eye rash or fever with accompanying muscle pain.

Disease transmission spikes in the summer, reflecting the peak in feeding by nymphal ticks and human outdoor activity. On the whole, the number of infections and the disease's range is increasing.

Could climate change be playing a role? To understand the impact of a warming climate on Lyme disease, it's helpful to reflect on the history of this tick-borne illness.

When Lyme disease was first discovered in the 1970s in southern New England and New York, scientists identified a new bacterium (Borrelia burgdorferi) that causes the disease. They also thought they had found a new species of tick that transmits the bacteria, which they gave the common name "deer tick" (scientific name Ixodes dammini).

The scientists then set out to describe the ecology of this "new" tick. They found them predominantly in coastal areas, and they found many of the ticks feeding on white-tailed deer, hence the common name. They concluded from these early studies that the ticks, and thus Lyme disease, were limited to climates near the coast.

At the same time, it was found that it was easy to kill ticks in the lab by subjecting them to extreme temperatures (low or high) and low humidity. Scientists began to suspect global warming might spread the warm, coastal conditions the tick preferred, causing Lyme disease to spread inland and to higher elevations. Ecological and climatic models also supported this hypothesis.

Evidence debunks theory

Three decades later, it's evident Lyme disease has, and is continuing to, spread dramatically in eastern North America. But the evidence now indicates the early models used to predict the disease's spread were too simple and sometimes flat-out wrong.

Why were they wrong and why is Lyme disease spreading? First, it turns out there is no such thing as a "deer tick." The tick found where Lyme disease emerged was not a new species, but rather a northern population of a species that has been known to science for 150 years.

This species is the blacklegged tick (Ixodes scapularis). Blacklegged ticks range from Georgia and the Carolinas to Texas and Oklahoma, up to Minnesota and Wisconsin and into the mid-Atlantic and New England states. Clearly, they can live in a broad range of climatic conditions.

Second, although it's true Lyme disease has moved inland, northward and to higher altitudes, as would be expected under climate warming, it has also spread south from coastal New England into Maryland and Virginia.

And third, blacklegged ticks are sophisticated little creatures that can seek protection from extreme climatic conditions in the soil or under leaf litter. So they're fairly well buffered against cold winters or dry summers. We know very little about which climate extremes are important causes of mortality.

So, what is the role of climate in the spread of Lyme disease? The short answer is, we don't yet know.

It's highly likely the geographic range of the blacklegged tick is limited at high latitudes and high elevations by extreme cold. And climate change is pushing northward and upward into the zone of extreme cold. So we should expect the geographic boundaries of Lyme disease to move northward and upslope.

But the spread over the past 30 years seems to have been caused more by the movements of host animals — especially larger mammals: raccoons, skunks and deer and migratory birds — which can move ticks miles in a few days.

So the emerging picture is that highly mobile hosts are constantly introducing ticks into new environments. Whether these new tick populations persist or die out depends in part on climatic conditions. Climate change is making their survival in northern, higher elevation areas more and more likely.

More research in this area is needed before we can confidently make predictions.

What we do know is that Lyme disease, which has been a serious problem in the Hudson Valley for decades, is now spreading throughout the state's northern and central reaches.